Bummer. The service manual also doesn't have the usual FRU part listing.
Based on what little information we do have, my hunch is that the q9400 would work because the e6700 pentium is 2 years newer than the q9400. Also, I have a gateway sx2803 that originally came with an e6700 pentium and I was able to upgrade it to a q9550. There was little information about this upgrade online as well. Good luck and report back what you find out!